1) Field of the Invention
The present invention relates to an improvement to a transmitter and a receiver of a spread spectrum radio communications system. More particularly, this invention relates to an increased transmission rate of a spread spectrum transmitter and a spread spectrum receiver employing an M-ary/SS system.
2) Description of the Related Art
In recent years, attention has been focused on spread spectrum (hereinafter referred to as “SS”) systems as a transmission system of information signals such as image signals and audio signals, in the field of mobile communications system.
Among such SS systems, an “M-ary/SS system” is adaptive for increasing the transmission rate and has been used widely. In the M-ary/SS system, mutually orthogonal 2K code sequences (hereinafter referred to as “orthogonal code sequences”) are stored in advance in both a transmitter and a receiver. The transmitter sequentially generates data sequences in unit of K bits (K≧2) from an information signal. The data sequences are respectively replaced by prescribed orthogonal code sequences related thereto in advance, and the orthogonal code sequences are multiplied by prescribed pseudo-noise (PN) codes, so that signal spectrum is directly spread for radio transmission. The M-ary/SS system can transmit a single orthogonal code sequence corresponding to an information signal of K bits, by one period of PN code as a minimum unit in spread spectrum modulation process.
FIG. 11 shows in block diagram a transmitter of a conventional M-ary/SS system disclosed in U.S. Pat. No. 5,103,459. The operation of the transmitter of the conventional M-ary/SS system will be explained below with reference to FIG. 11.
First, a data generator 1 generates an information signal to be transmitted. The data generation rate of the information signal will be referred to as “bit rate” Rb (bits/sec).
A serial-parallel converter 91 sequentially converts the information signal into parallel data sequences of an information length of K bits, where K is 2 or greater natural number. The number of parallel data sequences generated per unit time will be referred to as “symbol rate” Rs (=Rb/K), and the generation period of parallel data sequences will be referred to as “symbol period” Ts (=1I/R, unit: sec).
A Walsh function converter 92 receives parallel data sequences, and generates, from among a total number of 2K orthogonal code sequences predetermined for an entirety of an associated spread spectrum communications system (Walsh sequence defined by Walsh function and a respective orthogonal code sequence has a sequence length of 2K bits), those orthogonal code sequences related in advance to the parallel data sequences, and output the generated orthogonal code sequences. The generation rate of the orthogonal code sequences is identical to the symbol period Ts of the parallel data sequences.
A PN code generator 6 generates PN codes with a code cyclic period of L chips, a chip generation rate Rc(=L×Rs, unit: chips/sec, hereinafter called “chip rate”), and a chip generation period Tc (=1/Rc, unit: sec, hereinafter called “chip period”). It is assumed herein that the cyclic period L chip of the PN codes is an integral multiple of the sequence length (2K bits) of the orthogonal code sequences, and that a specific PN code is allotted in advance in each transmitter.
Next, a spreading modulator 93 multiplies the orthogonal code sequences by PN codes to generate SS signals.
FIG. 12 shows in time chart generation timings of the SS signals. In FIG. 12, the sequence length 2K of orthogonal code sequences=4 bits (K=2), and the cyclic period L of PN codes=8 chips (the number of chips of PN codes per one bit of orthogonal code sequence L/2K=2).
A frequency converter 9 performs a frequency conversion process by multiplication of the SS signals and a carrier of a prescribed frequency, and a power amplifier 10 performs power amplification of the SS signals after the frequency conversion process to generate a radio signal. A transmission antenna 11 transmits the radio signal.
As explained above, the transmitter of conventional M-ary/SS system spreads spectra of an information signal to be transmitted by using a total number of 2K orthogonal code sequences predetermined for the entire spread spectrum communications system and spreading codes (PN codes) specific to respective transmitters.
The conventional M-ary/SS system has a transmission rate tr (bits/sec) defined by the following equation 1.                               t          r                =                              K            L                    ⁢                      R            c                                              (        1        )            
In the convention M-ary/SS system, however, as one PN code is allotted to one transmitter, the transmission rate of transmitters is restricted to the transmission rate tr defined by the equation 1.
By allotting a plurality of PN codes to one transmitter to perform a plurality of radio communications in parallel using the PN codes, the transmission rate can be increased. However, this results in an increase in scale of circuitry of associated transmitter and receiver.
For a limitation to the number of PN codes concurrently employable within an entire spread spectrum communications system, the allotment of a plurality of PN codes to a single transmitter results in running out of employable PN codes.